Blue Grass Airlines
Feature Article for July 2003
Kevin Johnson, BGAS005
Flight Sim and the Art of Celestial Navigation
One of the most boring things you can do in flight-sim is take a prop liner on a long over water leg. Flying at 18 – 20 thousand feet at 250 kias (290 ktas), it can take quite a while to get from LaGuardia to Gatwick. Seems like there is nothing to do but read a book, or go to bed and check on it in the morning. Well, if you want to invest the time, there is something you can do to liven up the time over water.
Before the age of GPS and even before the introduction of the Inertial Navigation System (INS), long range transport aircraft had a crew position for the navigator. On long over water flights, the navigator was busy throughout the entire journey, carefully monitoring the position of the aircraft as well fuel and weather. He was responsible for accurate position reports that would be radioed to the oceanic control agency monitoring the progress of the aircraft across the long stretch of ocean. Often the only tools he had was a hand held sextant, a compass and an accurate watch.
The navigator began preparing for an over water flight at least a day in advance. He was responsible for obtaining a weather briefing and planning the route to fly to the destination. Once he received the forecast weather report and winds aloft, he could complete his flight plan and brief the pilot and engineer on fuel requirements and any significant concerns that might impact the flight.
The information for this article is based on my experience as a KC130F navigator with the Marine Corps during the early 80’s and may not reflect civilian airlines procedures during the 50’s. However, the techniques described in the following pages should reflect the techniques that would have been used by any navigator during the period from 1940-1980. Unfortunately, I have had to abbreviate, abstract and just plain leave out many things due to time and the limits of the Flight Simulation itself.
· Ready to taxi, late afternoon departure.
Preflight Planning Checklist:
[ ] Request the weather package and receive the preliminary weather forecast.
[ ] Determine the route or “red line” and plot out on the navigation chart.
[ ] Finalize the flight plan and “spin the winds”
[ ] Determine time enroute and calculate the fuel required based on fuel consumption of the aircraft at an estimated TAS and altitude.
[ ] Construct “howgozit” chart to monitor fuel consumption. (Because of time constraints this will not be covered in detail. However, if anyone is interested just e-mail me and I’d be happy to provide some more detailed information on what this entails.)
[ ] Calculate Max Gross Take Off Weight (MGTOW) based on passengers, cargo, required and reserve fuel to establish take off speeds.
[ ]Ensure sextant, enroute charts, navigation charts, flight logs, DR kit, etc are completed. (program HFADF gauge in flight simulator).
Determining the route and plotting the Charts:
The navigator uses “charts,” not maps, to determine the geographic location of the aircraft. There are many different charts available. I would suggest using a Global Navigation Chart (GNC). They are large, approximately 3’x 4’ and are nice to pin up on the wall if you have room. These charts are reasonably priced and may be ordered on line or purchased at pilot supply shops. For flights across the North Atlantic use a GNC3. The GNC3 covers the northeast US and as far east as Russia.
The GNC is a Lambert-conformal projection which means the lines of longitude and latitude are perpendicular. This allows for the True Course (TC) to be measured relative to the line of longitude. Distance is measured in latitude along the lines of longitude. Each degree of latitude is equal to 60 nm and contains 60, 1 nm subdivisions called “minutes.” So, for example measuring the distance from 30 degrees north latitude, along the line of longitude, to 33 degrees north latitude equals 180 nm (3 x 60 = 180). However, any distance may be determined by taking the distance between two points with a set of dividers and measuring the distance on a line of longitude. Another advantage of the Lambert-conformal projection is that great circle routes appear as a straight line. This makes flight planning of direct routes much easier.
Another crucial piece of information represented on the chart is Magnetic Variation (Mag Var). Mag Var is the angular distance from a point on the globe between the magnetic and geographic north poles. Navigation charts are aligned to TRUE north, which is the geographic North Pole or 90 degrees north latitude. However, your compass points to the MAGNETIC north pole. The difference between True Course (TC) which is used on the chart, and Magnetic Heading (MH), is Magnetic Variation. Understanding the difference is absolutely crucial to the navigator. Hopefully the illustrations and the logs will make this clear. For example, mag var in Miami is 4 degrees west, while it is 16 degrees east in San Francisco. That is a 20 degree difference across the US! This means that the angular distance between what you plot on the chart, which is called True Course (TC) may be up to 20 degrees different than the Magnetic Heading (MH) shown on your compass! The way to correct for this is shown in the following formula:
Western magnetic variation is ADDED to True Heading or True Course.
Eastern magnetic variation is SUBTRACTED from True Heading or True Course.
TC +/- Drift = TH, TH +/- Mag Var = MH
Note: Drift is the correction for the wind aloft. For this example, we will not program in any wind. This will simplify navigation considerably. Again, if anyone is interested, I would be happy to deal with the complexities of including the wind calculations. There is also compass deviation, which is error in the specific compass you are using which gets you from MH to Magnetic Course (MC), but that is even too fussy for me and will not be considered!
Example: The True Course (TC) measured off the chart to hit the next waypoint is 85 degrees. What is the magnetic heading to give the pilot to fly on the compass? Assuming mag var = 12 degrees west (read off the chart) and 0 degrees drift due to wind (Again, I will not even attempt to explain calculating the drift due to wind at this time.)
TC=85 -0 drift = 85 degree TH. 85 TH + 12 degrees west mag var = 97 degrees MH.
Fly 97 degrees on the compass. (See the attached flight log for more examples.)
On the charts I am using, the mag var is from 1980! So it was necessary for me to correct it to 2001 mag var for FS2002. The progression rates are printed on the chart itself, so it is easy to do. They had only changed 2 degrees in 21 years. This doesn’t not sound significant, but each degree of error translates to 1 nm off course for each 60nm traveled. Assume you are 2 degrees off, traveling at 300 knts Ground Speed (GS) for an hour. Over 1 hour, you could be 10 nm off course and not know it. Multiply this by seven hours of over water flight time without a fix to confirm your location (it happens!) and now you are looking at 70 nm off course, without any other error creeping in!
Getting back to the flight plan. List all major way points and navigation aids and the distances and headings between them. This will allow you to estimate the time enroute and the amount of fuel needed. This can be crucial if flying into a head wind with limited amounts of fuel. For our flight, however, we will assume a no wind condition. With max fuel at FL200, we should have around 14 hours of flight time and an almost 4000nm range in our L1049g. Our trip to Gatwick should take around 12 hours and is approximately 3100 nm. With a 50-60 knt tail wind we could make it about 10 hours. Or conversely, with a head wind, the trip would take about 14 hours 30. This would be typical when coming back across the Atlantic. This would make the crew watch the “howgozit” closely to see if a fuel stop in Shannon or St. John’s was necessary. (Returning from the Azores once in a C130 at FL 230, our ground speed was below 200 knts for most of the trip! We normally cruised at around 305 KTAS.)
To continue, determine the last navaid to be used, then determine where you will pick up jet routes again after crossing the ocean. Draw a straight line between these two points. This will give you a great circle route, which is the shortest transoceanic route you can fly. However, based on weather reports or NOTAMs, you may need to consider a different route.
For our example flight from LaGuardia to Gatwick, our last navaid will be St. John’s. The navigator will not earn his pay until the needle starts spinning on the VOR! We will be aiming for Land’s End on the other side of the Atlantic and fly a great circle route in between. See sample below:
After flight planning and obtaining the estimated time enroute, it is time to determine the amount of fuel needed.
MGTOW: 137,500# (found in the aircraft.cfg file)
Empty Weight: 73,856#
Max Weight w/ payload: 103,460#
Max fuel: 43,190#
Max fuel with max payload: 34,040#
Max landing weight: 112,340# (estimated at 82% MTOW)
Est. 330 gal/hr ( 6 lbs./gal) = 1980 lbs/hr at 20,000 ft, 236 kias
This will provide approximately 4300nm range. For our flight, with reserve, alternate, holding, taxi, run-up, etc. We might as well just fill ‘er up. Like the old saying goes, “there’s nothing worse than runway behind you, altitude above you and fuel you don’t have when you need it!”
(Note: This is nearly 1000nm farther than the range normally indicated for a L1049g.)
The extent of the navigator’s job:
The navigator’s job really begins in earnest as the aircraft is reaching the range limit of the land based radio aids (VOR, TACAN, NDB, ADF). His first real job will be to provide the pilot with a good initial heading to fly after the navaids quit. Once out over the water, he will be responsible for making sure the aircraft gets safely to the other side of the pond. The navigator will keep the pilot and rest of the crew informed of the aircraft’s progress and location. He will also aid in any decision making concerning diverting the aircraft for fuel, weather or mechanical problems. The navigator’s job is essentially complete when he returns navigation of the aircraft back to the pilot-in -command as the aircraft approaches land and the radio aids become active again.
“We’re not lost. Just off course.”
One of the main techniques used by the navigator for keeping track of the aircraft’s location is by dead-reckoning or DR. “Dead” is really an abbreviation for “deduced” reckoning. This is the technique of plotting the current True Course (TC) and Ground Speed (GS) of the aircraft to determine where the aircraft will be in a given period of time. Usually as part of the navigator’s “routine” a DR position and a “fix” will be determined at least every hour (see notes below on the navigator’s routine). Note, the DR plot is not a “fix.” Rather, it is where the navigator assumes the aircraft’s position will be in a given amount of time. During the day, a DR plot and celestial fix should be obtained as frequently as every half hour. At night, the DR plot and celestial routine could be extended to once an hour. The DR position is important because there may be times when the navigator has no navaids or Lines Of Position (LOP) available to determine the aircraft position! Overcast skies, unreliable LORAN and no radar altimeter for Pressure Pattern (PP), can leave the nav with nothing to work with except a compass, watch and the 24 hour old forecast winds! Believe me, I’ve been there!
The DR kit needs to contain:
A set of dividers (drafting type okay – with points)
Aviation plotter (available at aviation supply store, but a protractor could do in a pinch)
A drafting template with triangle, square and circle for plotting positions on the chart.
A calculator for determining time/speed and distance problems (Jepson makes these)
The navigator’s routine:
1. Obtain and plot LOPs.
These may be by: Radio Navaids, RADAR, LORAN, or Celestial.
2. Determine aircraft location based on LOPs. Record Lat/Long in Nav Log.
3. Determine the current drift, TC and GS. Record in Nav Log.
4. Plot the next DR location based on the aircraft’s current position, GS and TC for the time of the next fix.
Ex. 087 degrees TC, 265 nm for 1 hour.
5. Plot the “no wind” position based on TH and TAS. (If no winds aloft programmed into FS, the “no wind” and DR plot will be identical)
6. Make any required course correction to keep the aircraft within 20nm of its flight planned route. (Be sure to account for change in mag var by using the AVERAGE mag var the aircraft will be passing through before the next fix.)
7. Update any changes to ETA on position reports or destination.
8. Record fuel consumption. Update the “howgozit.”
9. Select celestial objects for next “shot” and precompute their assumed positions.
This routine is somewhat abbreviated since the majority of calculations needed to make a celestial “shot” using the sextant are being simulated.
Navigation and the LOP:
“Nav, Pilot. What’s our position?”
“This is nav. Just over the “A” in Atlantic.”
The navigator has numerous navaids available during the flight. While overland and in radio range, VOR, NDB, TACAN and ADF will be the primary navaids. However, if the aircraft is equipped with radar, this may also be used. Once over water, and out of radio range, a different set of navaids become available. LORAN, Pressure Pattern, RADAR and Celestial. Celestial Navigation, utilizing a hand held or periscopic sextant to make observations of the sun, moon, planets and stars is the Navigator’s primary tool. However, he should aggressively take advantage of whatever is available! If you can look out the window and see a ship or an island, use it!
VOR, NDB, ADF
I’m sure most of you are very familiar with the use of these navaids. The only tricky part is that converting the radials to TRUE so that you can plot them on the chart (which is aligned to TRUE NORTH verses MAGNETIC).
This will be one of the primary tools for simulating Pressure Pattern and Celestial Navigation. Why bother? Just for the fun of getting a very small flavor of the challenges facing the navigator of an aircraft back in the 40’s, 50’s and 60’s.
To get a radar fix, the navigator could tilt the aircraft’s radar beam down slightly to pick up ground returns. A typical weather radar in 1959 had a max range of approximately 60-80nm. A RADAR fix may be simulated by going to the map mode and zooming out to an appropriate distance. Try zooming out to where the screen switches to “North up” and use the “-“ key to back out four times. This would represent a “max range” of approximately 80 nm. Each increment approximately doubles the range. So, the initial “north up” would show approximately 5 nm range. Once out, 10nm. Twice out 20nm. Three times out, 40nm. Five times out 80nm. A real radar screen has a cursor to read relative and magnetic headings, but for FS you will just have to eye-ball it. Still, this can be handy as you are approaching the coast or an island with prominent features.
The navigator’s primary means of determining the aircraft’s position over water is with a sextant. This could be either a hand held or periscopic sextant. The navigator determines which celestial objects are visible and best suited to provide a Line of Position (LOP) for the aircraft. During the day the sun is usually available unless obscured by weather. But the moon, Venus and even Jupiter are sometimes visible and can be used to determine your position! Obviously, at night there are more stars to choose from. Polaris (the North Star) is extremely helpful in the northern hemisphere because its angle above the equator accurately provides current latitude. The LOP is determined by finding the angle of the celestial object above the horizon for a specific time at an assumed location on the earth’s surface. Obviously, I am glossing over a lot. Celestial theory is not overly complicated, but explaining it thoroughly can be very time consuming! So, I’m just sharing enough of the basic theory to explain how I simulate it. Basically, the distance from your location to the actual point where an imaginary line passes from the center of the earth to the star and intersects the earth’s surface is what gives you your line of position. This translates into a straight line that may be plotted on the chart.
Depending on the location of the celestial object relative to the heading of the aircraft, the LOP obtained will be either a “speed line” or a “course line.” The speed line is perpendicular to your course, showing your speed. The course line is parallel with your course and will show you if you are left or right of your intended flight path. A single LOP will give you one or the other. Ideally, you will have two or more LOPs to determine or “fix” your position. It is best to have both a “course” and a “speed” line. At night, numerous stars are available for navigation, making the navigator’s job easier. However, dark and stormy nights crossing the Atlantic often left the navigator with nothing to see in his sextant! During the day, often only one celestial LOP (the sun) is available. Depending on the length and direction of your flight, the sun will turn from a course LOP to a speed LOP as it moves across the sky during the course of the day.
Besides Celestial navigation, a fascinating and surprisingly reliable LOP could be obtained called “Pressure Pattern” (PP). PP measures the drift that naturally occurs on an aircraft as it moves from one area of pressurized sky into another. If moving from a high pressure system to low in the northern hemisphere, the aircraft drifts to the right. If moving from a low pressure system into a high, the effect is the opposite. This is due to pressure differential and Coriolis effect. (You know, the phenomenon that makes the water drain clockwise above the equator and counter clockwise below. Or is that the other way around?) By recording the difference between pressure and actual altitude over a specific period of time, drift could be calculated, left or right of course. This results in a course LOP and requires a functional radar altimeter to work. (If anybody remembers “The High and the Mighty” they show the navigator using one of these. This happens to be an old favorite of mine. I personally used this method regularly as a nav. We called it the “green worm” because the altitude was read off of a squiggly green fluorescent line on the altimeter.) Below are sample logs showing fuel readings for the “howgozit” and LOP data for plotting a fix:
Using the HFADF gauge for celestial and pressure pattern LOPs:
This gauge is the basic tool that I use for simulating celestial navigation in FS. The HFADF gauge is intended to simulate a long range radio aid. The gauge allows you to program in 20 different stations. However, with a little imagination, it can be used to simulate celestial and PP navigation by representing the LOPs that can be obtained by using a sextant or radar altimeter.
The first thing to do is to add the gauge to whatever panel you are using. I am currently using the new Connie panel that is on Tom Gibson’s awesome website. I have located the HFADF on the Flight Engineer’s Panel in the following location: gauge65=fakeadf, 63,14, 75
The gauge itself looks like an ADF. The needle points to the location preprogrammed into the gauge prior to starting FS. Clicking on the number appears in the upper right corner of the gauge cycles you through the preprogrammed points. The needle is somewhat difficult to read, but if it is straight ahead, then it equals the current MH of the aircraft. If it is left or right, you must determine the difference between what it reads and your MH to determine your bearing or LOP to that point. Don’t forget, Mag Var must be included to change from MH back into TC. This gives you a bearing to plot from the position of the “star.” It is important to program points to simulate the position of stars that provide both speed and course LOPs for the entire flight. Additionally, you can simulate a day flight by only programming one or two points in to represent a single LOP from the sun.
Programming the HFADF:
For our flight I selected and programmed the following points to simulate the naviads available for the flight:
Pos: Name: Coordinates:
1 HFADF1 50 08 N 05 41 W (Land’s End)
2 PP 51 00 N 30 00 W (Provides course LOP for entire flight)
3 Cel2 55 00 N 20 00 W
(Provides a speed LOP for second half of flight: the rising sun as we approach UK)
4 Cel3 48 00 N 30 00 W (Provides a speed LOP for middle of flight)
5 Cel4 52 00 N 40 00 W (Provides a speed LOP for the first half of flight)
Using the HFADF and plotting a fix:
PP is located at 51N 30W. This is very close to the middle of the flight and rests directly on our plotted course line. If you get left or right of this point, it represents the drift associated with pressure pattern navigation.
At 2100L we recorded the following information:
PP, HFADF position 2, read 2 degrees right of MH (MH +2).
MH = 099 + 2 = 101 degree relative bearing to PP location.
Convert PPs Mag LOP to True so it can be plotted on the chart.
101 MH -20 degrees West Mag Var = 81
degrees. Since your DR plot is to the west of PP, add 180 degree = 261 degree TH.
Draw a line in pencil from PP, 261 degrees towards your DR.
· Part of GNC3 showing the course line, first two over-water fixes and DR position anticipating the third fix.
Okay, let me say I’m impressed that you are still reading! I admit this is a bit much to digest. So maybe you’re thinking, “Why not just fly straight to the HFADF point?” Or better yet, “just use GPS.” Fair enough. But doing it the “old fashioned” way adds some realism and challenge! This probably isn’t going to appeal to everybody, but to me it captures a little of the flavor of long, lonely nights over the cold Atlantic when the navigator was anxiously keeping the airplane and a hundred sleeping passengers on course and safe. Okay, maybe it is a bit much and maybe a bit too “romantic”. But hey, it’s all fun, especially if you like all the math!
Right, back to the text book. With only one LOP, you can’t get a very accurate fix because all you know for sure is that you are somewhere on that line! This is where an accurate DR is crucial. Plot the single LOP then draw a line perpendicular form the DR plot through the LOP. Where they intersect is your best estimated current location.
Now, if you are using more than one LOP, cycle through their positions on the HFADF and write down the relative bearings of the ones you need. Then plot them out. This will reduce the error of the aircrafts position changing as you plot your fix. Try to take your readings as close to the time of your “fix” as possible.
To continue the previous example, since this is a night flight assume you have two celestial LOPs in addition to a working Radar altimeter for PP. We will use Cel3, Cel4 and Pressure Pattern for this fix. Cel2 and HFADF are too far away to of much use.
Cel3, HFADF position 4, reads 19 degrees right of MH.
MH = 099 + 19 = 118 degrees relative bearing to Cel3 location.
Convert Cel3’s Mag LOP to True so it can be plotted on the chart.
118 MH -20 degrees West Mag Var = 98 degrees. Since your DR plot is to the west of Cel3, add 180 degree = 278 degree TH.
Draw a line in pencil from Cel3, 278 degrees towards your DR. You now should have two lines that cross. This is enough to give you a fairly accurate fix, but since another LOP is available use it.
Cel4, HFADF position 5, reads 36 degrees LEFT of MH.
MH = 099 - 36 = 63 degrees relative bearing to Cel4 location.
Convert Cel3’s Mag LOP to True so it can be plotted on the chart.
63 MH -22 degrees West Mag Var = 41 degrees. (Notice the mag var is different than before, this is because Cel4’s position is nearly 300 nm away from the other two.) Since your DR plot is to the west of Cel4, add 180 degree = 221 degree TH.
Draw a line in pencil from Cel4, 221 degrees towards your DR.
Now you should have a triangle formed by the three LOPs. The smaller the triangle the better. Your position, as of the time the readings were taken, is assumed to be at the center of the triangle. Below is a portion of a flight log showing a variety of fixes:
· Flight log showing actual positions and times
Note: Be very careful with the mag var so that you don’t plot it going the wrong way! Remember, West is positive, or added, when converting from TRUE to MAGNETIC. It is negative, or subtracted when converting MAGNETIC to TRUE. However, it should be pretty obvious since the LOP will be way off of where you expect it to be.
Now measure the distance between this fix and the last one and you can determine GS and wind drift. Plot your DR for the next fix time then go get a cup of coffee until the routine starts over! This will keep you pretty well occupied until the Radio aids lock on again. Then the sextant can be stowed and navigation of the aircraft turned back over to the pilot. You will know you are doing your job as the nav, if at the time you estimate you should be back in radio range, the VOR locks on and the needle is pointing directly on your nose!
Note: I do everything in real time until I get out over the water. Then I do the nav routine in real time. But, after the DR for the next position is plotted I speed up to x4 until the time of the next fix. This shortens the time commitment considerably without sacrificing (in my opinion) too much realism.
· Night over water.
Passing position reports.
One last thing, while over water, part of the navigator’s job is to provide the position reports to be passed to the Oceanic Control Agency. Positions are tracked on the flight log in Latitude and Longitude. Also estimated times to the upcoming way points and the destination are also logged. Normally the navigator must update ETA with Center if arrival time is going to change by more than 30 minutes. The co-pilot or radio operator was responsible for actually passing the reports when I was flying on C130s.
Flight __________ over _______________/ ______________, at ___________z.
Estimate _______________/ _______________ at ___________ z.
I realize that it is virtually (no pun intended!) impossible to completely condense seven months of Navigation school plus 1000 hours of over-water flight time in a single article. For example, I entirely left out correcting for the winds aloft, maybe in another article. But hopefully, what I’ve provided is enough to get you interested. I know this was a lot to absorb and pretty confusing. Therefore, if this has wet anyone’s appetite and you would like some additional information, please e-mail me at firstname.lastname@example.org. I will do my best to clarify. I also know that there are many of you out there with enough experience in both flight sim and the real world to put me to shame. So if you have comments or suggestions on how to do this better, I’d like to hear them.
My suggestion is take your time, don’t try it all at once and cheat if you want! Don’t forget that “Shift Z” will give you your location in latitude and longitude. I call this a LORAN fix! I’ve flown long over water legs using this technique a dozen times in FS, with B314, B377, DC4, L1049, DC8, B707 and even a CV880. One draw back is that it takes a while to flight plan. However, I find that I am a lot more mentally “invested” in the flight, making the experience much more enjoyable. Besides, seeing a GNC spread out, with the dividers and plotter, helps to transforms my earth bound desk into a flight station out over the ocean! But more pragmatically, I can actually be doing something rather than just baby sitting, while the plane flies on auto pilot. I think it also makes the hours logged more valuable and the vicarious experience of FS more enjoyable too! Finally, in a limited way, it helps recreate a lost art that was once an essential part of any transoceanic flight, going back to the great age of classic aviation simulated by Blue Grass Airlines.
“Pilot to Nav, what’s our initial heading?”
“Second star to the right and straight on till morning!”
· Morning twilight over the Atlantic – a short night!
Required tools (available at aviation supply stores):
A plotter ( A protractor can also be used)
Charts – Global Navigation Charts (GNC3 covers the North Atlantic)
Red pen, pencil
A flight computer (I use a Jepson hand held “whiz wheel”, but time/speed/distance can be done long hand with a calculator.)
A drafting template with circles, triangles and squares.
Download and install HFADF (fakeadf.gau) gauge from AVSIM or other flight sim site.